The Gasoline Direct Injection (GDI) is available on a variety of late model engines: Audi, BMW, GM, Ford, Hyundai, Lexus, Mazda, MINI, Nissan, Porsche, VW and others. GDI sprays fuel directly into the combustion chamber under great or high pressure, rather than spraying fuel under low pressure into the intake ports in the cylinder head. GDI increases fuel economy and power 15 to 25 percent, but there is a downside that is now getting apparent as these engines accumulate miles.
The issue is carbon deposits are building up on the inlet side (top) of the intake valves. The deposits produce turbulence and can restrict airflow into the cylinders creating performance and driveability issues (hesitation, stumbling, misfiring, even hard starting). The thicker the carbon deposit buildup on the valves, the worse the driveability issues.
GDI sprays fuel directly into the combustion chamber so the fuel completely bypasses the intake valves. Consequently, detergents and cleaners that are included in gasoline to prevent intake valve deposits from forming in port fuel injection engines never have a chance to do their job in a GDI engine. The inlet side of the intake valves are never in direct contact with the fuel so the detergents cannot wash away the deposits. As a result of this, fuel detergent additives that are either in gasoline from the refinery or are included in the fuel tank have almost no effect on stopping or removing intake valve deposits in GDI engines. The additives functions in regular fuel injected engines, but not GDI engines.
What Causes Intake Valve Deposits
Intake valve deposits form as a result of oil slowly seeping past the intake valve guide seals and down the valve guides. A tiny quantity of oil is required to lubricate the guides, but when oil reaches the hot surface of the valve, it can stick and burn forming heavy black carbon deposits that gradually build up over time. The higher the mileage on the engine and the greater the wear in the valve guides and seals, the faster the accumulation of black carbon deposits on the intake valves. Low viscosity motor oils (such as 5W-20 and 0W-20) may make the issue worse because they are thinner (to reduce friction) and flow more easily down the valve guides. Conventional motor oils equally have a lower flash point than synthetic oils, which can equally increase the formation of deposits over time.
Another responsible factor to the formation of intake valve deposits is unburned fuel vapors and oil vapors being siphoned back into the intake manifold through the Positive Crankcase Ventilation (PCV) system. This is performed to control crankcase emissions and to remove moisture from the oil (which helps prolong oil life). The fuel vapors, carbon particles and oil droplets that the PCV system routes back into the intake manifold are reburned in the engine to lower pollution. But these same vapors can also form carbon and varnish deposits on the intake valves.
The more blowby an engine has due to cylinder and piston ring wear, the greater the volume of crankcase vapors that are pulled back into the engine by the PCV system. High mileage engines typically have more blowby than low mileage engines, so the build up of intake valve deposits is often faster.
Diagnosing Intake Valve Deposits:
An engine that is having driveability and performance problems as a result of intake valve deposits may or may not set any Diagnostic Trouble Codes (DTCs) and turn on the Check Engine light. If the engine is misfiring bad enough, it may set a P0300 random misfire code or individual cylinder misfire codes. However, many other factors can equally set misfire codes so a misfire code alone is not necessarily a sign the engine has dirty intake valves.
You cannot see intake valve deposits directly due to the valves are inside the cylinder head. The only way to see deposits on the intake valves is to remove the intake manifold and peer into the intake ports in the cylinder head – unless you have a fancy tool like a boroscope or a fiber optic video camera that can be put into the combustion chamber through the spark plug hole or snaked down the intake manifold to inspect the valves. Few automotive technicians have this kind of equipment and probably wouldn’t use it even if they had it since they would proceed with the assumption that the valves are dirty and need to be cleaned.
How to lower carbon deposits on GDI Intake Valves
How fast the intake valves get dirty does not seem to be a duty of fuel quality or how much ethanol alcohol is in the gasoline. Rather, it seems to be influenced most by how often the engine oil is replaced. Oil vapors and combustion byproducts that are taken back into the intake manifold through the PCV system seem to add most to carbon deposits on the intake valves.
My advice is to replace your oil every 3000 miles if you only do short trip stop-and-go city driving, or change your oil every 5000 miles if you do mostly highway driving. If you want to minimize carbon buildup on the intake valves, don’t push your oil change intervals to 7500 miles or longer unless you are using a high quality full synthetic oil (which usually has less volatility than conventional motor oil).
Replacing your oil regularly will help minimize the carbon buildup on the valves, but eventually they may still get dirty. If that occurs, it may be necessary to clean the valves every 25,000 to 30,000 miles with an aerosol cleaner that is sprayed into the intake manifold.
How to Clean Dirty Intake Valves
If you feel the intake valves on a GDI engine are dirty but you do not want to go though all the work of removing the intake manifold and cylinder head, you can attempt to clean the valves using the following procedures:
Get a bottle of liquid engine top cleaner, intake system cleaner or carburetor cleaner to clean the intake valves. Follow the directions on the product, or proceed as follows:
With some products, the cleaner is sprayed into the throttle body while the engine is running. Others recommend disconnecting the PCV hose from the PCV valve, or using any other large vacuum hose that links to the intake manifold so you can slowly pour the cleaner into the hose while the engine is running (you will probably need a small funnel for this). Run the engine at fast idle (say 1000 to 1500 RPM) while feeding the cleaner into the intake manifold.
Depending on how dirty the intake valves are and how effective the cleaning chemical is, the process may take 10 to 20 minutes or more to remove the carbon deposits. You may equally have to repeat the cleaning process more than once to completely remove the deposits.
If this cleaning process fails to do the job due to the carbon deposits are so thick, you may have to try a more direct cleaning approach. This needs removing the intake manifold so the cleaner can be applied directly to the valves. You may have to refer to the factory service information for detailed step-by-step removal procedures for the intake manifold.
CAUTION: If you have to disconnected any fuel lines to remove the intake manifold, ensure all residual fuel pressure inside the lines has been relieved before opening any lines.
Once the intake manifold has been taken out, look into each port to see which valves are closed and which valves are open. The cleaning process will begin with all of the valves that are CLOSED. Once those valves have been cleaned, rotate the engine to close the remaining valves that were open. The reason you want the valves closed when you clean them is so the cleaning chemical and carbon deposits don’t fall down into the engine’s cylinders.
Use an aerosol product that can loosen and remove carbon such as brake cleaner.
Spray it cleaner directly into the intake port so that it puddles on top of the valve. Let is soak for about 30 minutes to loosen the deposits. You can equally use a small brush or pick to scrape at the deposits while the cleaner is working. After 30 minutes, soak up the cleaner residue with a rag or paper towels, and/or suck out the cleaner and carbon residue with a wet shop vacuum.
Accumulated carbon deposits on intake valves can prevent airflow. After cleaning, airflow is much improved.
Now rotate the crankshaft to close the remaining valves that are open, and repeat the cleaning process as required on the other valves that are now closed until all of the valves have been cleaned.
If the intake valve deposits are so thick and hard that chemical cleaning does not function, you can try blast cleaning the valves with an air gun blaster and soft blasting media such as walnut shells, baking soda or plastic beads. Seal or tape all other openings on the top of the engine so blast media and residue can’t get into the crankcase, coolant or oil passages. The blast residue can then get sucked out of the intake ports with a vacuum once the valves have been cleaned.
CAUTION: Ensure the valves are CLOSED before spraying blast media into the intake ports, and NEVER use any kind of hard blast media such as sand (silica), glass bead or metal beads as these can create severe damage to the rings and cylinders if any blast media gets past the valve.
If all else fails, the last option is to remove the cylinder head, disassemble all of the valves and clean them by hand with a wire brush, bead blaster or other blast media, or to soak the valves in a hot tank or ultrasonic cleaning tank.
Some vehicle dealers want to replace the entire cylinder head with a new one if the valves are badly carboned up. But this is an unnecessary expense since in most cases the valves can be disassembled, cleaned and reassembled in the existing cylinder head once the head has been removed from the engine. Disassembling a cylinder head and cleaning the valves takes more time and labor, and may need some special tools such as a valve spring compressor, gear puller or overhead cam removal tools. But it can save the cost of changing the entire head. The only time head change would be recommended is if the engine has a lot of miles on it (say well over 100,000 miles) and the head has other issues such as worn valve guides and/or seats, cracks or other damage.